The following references may be relevant to the present invention:
U.S. Pat. No. 5,329,259—Stengel, “Efficient Amplitude/Phase Modulation Amplifier”
U.S. Pat. No. 5,612,651—Chethik, “Modulating Array QAM Transmitter”
U.S. Pat. No. 5,659,272—Linguet, “Amplitude Modulation Method and Apparatus using Two Phase Modulated Signals”
U.S. Pat. No. 5,852,389—Kumar, “Direct QAM Modulator”
U.S. Pat. No. 5,867,071—Chethik, “High Power Transmitter Employing a high Power QAM Modulator”
U.S. Pat. No. 6,147,553—Kolanek, “Amplification Using Amplitude Reconstruction of Amplitude and/or Angle Modulated Carrier”
U.S. Pat. No. 6,160,856—Gershon, “System For Providing Amplitude and Phase Modulation of Line Signals Using Delay Lines”
U.S. Pat. No. 6,313,703—Wright et al., “Use of Antiphase Signals For Predistortion Training Within An Amplifier System”
U.S. Pat. No. 6,366,177—McCune, “High-Efficiency Power Modulators”
With the ever increasing demand for the high speed transfer of information digital systems are becoming very common. In its simplest form the modern telecommunication system requires circuits for modulation, frequency conversion, transmission and detection.
The basis for signal transmission is a continuous time varying constant-frequency signal known as a carrier. The carrier signal can be represented as S(t)=A cos(2πft+σ), where f is the frequency, A is the amplitude, and σ is the phase of the signal. S(t) is a deterministic signal, and alone carries no useful information. However, information could be encoded on S(t) if one or more of the following characteristics of the carrier were altered: amplitude, frequency or phase. In essence modulation is the process of encoding an information source onto a high-frequency, carrier signal S(t).
Bandpass digital systems can be divided into two main categories; binary digital systems or multilevel digital systems. Binary digital systems are limited in that they can only represent a one bit symbol (0 or 1) at any given time. The most common binary bandpass signal techniques are Amplitude Shift Keying (ASK), Phase Shift Keying (PSK), and Frequency Shift Keying (FSK). For example, a binary digital system using ASK might have a signal range from 0 to 3 Volts. Any value less than 1.5 Volts would represent a digital 0 and anything greater than 1.5 Volts would represent a digital 1. Alternatively, FSK would use two different frequencies and PSK would use two different phases to represent a digital 0 or 1. However, binary digital systems are not as practical as multilevel systems since digital transmission is notoriously wasteful of RF bandwidth, and regulatory authorities usually require a minimum bandwidth efficiency.
With multilevel digital systems, inputs with more than two modulation levels are used. In cases like this multiple bits can be sent with each symbol, increasing the speed and efficiency. In keeping with the previous example of an amplitude modulated signal with a range from 0 to 3 Volts, the signal amplitude could be broken into 4 distinct points; 0.75V could correspond to binary 00, 1.5V corresponds to 01, 2.25V corresponds to 10, and 3V corresponds to binary 11. In this case each symbol represents a two bit binary number. Alternatively, such transformations can be implemented by adjusting the phase or frequency of the carrier.
More advanced techniques for a multilevel digital system would include a combination of amplitude and phase modulation of a carrier signal. In this case a single multi-bit symbol could be represented by a signal with a certain phase and amplitude. Each symbol of digital data could be defined as a vector with a specified amplitude and angle and visualized on a polar axis. In one of its simplest forms a three bit digital symbol could be represented by two distinct amplitudes and four distinct phases.
There are various common modulation techniques which require the amplitude and phase adjustment of a carrier signal. Solutions to these modulation techniques are typically built in either analog or digital circuitry. One such solution which is shown and described hereinafter will be recognized by those skilled in the art as a IQ modulator. Due to its requirements for digital to analog conversion and linear power amplification before transmission, modulators of this form typically consume lots of power.